The present invention relates generally to polyolefin foams and, more particularly, to extruded foam sheets comprising a blend of low density polyethylene and ethylene/alpha-olefin copolymer.
Polyolefin foams, particularly those made from polyethylene, and methods of manufacturing polyolefin foam sheets are well known in the art. See, e.g., U.S. Pat. Nos. 5,462,974 (Lee), and 5,667,728 (Lee), the disclosures of which are incorporated herein by reference thereto. One of the most common polyolefins used to produce foam is polyethylene and, specifically, low density polyethylene (LDPE). While LDPE possesses a number of beneficial physical and chemical properties when used to produce a foamed sheet, a disadvantage of LDPE is that extruded foam sheets made therefrom have a tear strength that is lower than would otherwise be desired for certain applications.
U.S. Pat. No. 4,738,810 (Cheng-Shiang) teaches that the tear strength and other mechanical properties of an LDPE foam sheet can be improved by blending linear low density polyethylene (LLDPE) with the LDPE. The '810 Cheng-Shiang reference also teaches that the melt index (MI) of the LLDPE should be below 10 g/10 minutes, and preferably between 1 and 7 g/10 min., and that the foam sheet is cross-linked after extrusion.
The inventor hereof has found, however, that when an LLDPE with a MI of less than 10 g/10 min. is blended with LDPE, the shear forces exhibited by the blend increase sharply during extrusion, relative to extruding LDPE alone, resulting in the generation of excess heat. This heat generation was found to degrade the appearance of the resultant foam and lead to inferior mechanical properties due to a high percentage of open cells and non-uniform cell-size and cell-wall thickness. Also, the excess heat reduces the foaming efficiency, i.e., the foam has less void volume and therefore higher density for a given amount of blowing agent.
The foregoing problems are believed to occur because the heat build-up reduces the melt strength of the polymer melt and increases the volatility of the blowing agent, resulting in less void volume and more open cells as the blowing agent escapes from the polymer melt before it can cool sufficiently to form an adequate number of closed cells within which the blowing agent would otherwise be trapped. The increased heat is also believed to reduce the beneficial tendency of the LDPE to exhibit strain hardening during extrusion through a foaming die and during expansion outside of the die. As is known, strain hardening during extrusion and expansion promotes uniform cell-size distribution. One of the reasons that LDPE is commonly used in the production of foams is its strain-hardening capability and resultant promotion of cell uniformity. However, this capability decreases with increasing temperature, as caused, e.g., by the high shear forces exhibited by low MI LLDPE (less than 10 g/10 min.) during extrusion foaming.
Accordingly, a need exists in the art for a polyolefin blend capable of producing an extruded foam sheet with improved mechanical properties but without the drawbacks of using low MI (less than 10 g/10 min.) LLDPE.